The present invention relates to a method of controlling a turbocharger that generates a charging pressure in an intake plenum, the charging pressure being controlled on the turbine side. The turbocharger has a circulating-air valve that temporarily opens a flow cross-section between the intake plenum and a suction side of the turbocharger when an air mass flow flowing out of the intake plenum is reduced. The present invention also relates to a control device that controls the process sequence
Turbochargers with one or more circulating-air valves are used in the case of Otto engines that are operated in a throttled manner at least in certain operating conditions. The throttling is achieved by a reduced opening cross-section of an air mass flow adjusting member between combustion chambers of the internal-combustion engine and the intake plenum. The position of a throttle valve or an opening lift of intake valves, for example, can be used as an adjusting variable.
In the case of a high air mass flow rate, as it occurs as a result of a high rotational engine speed and/or high values of the combustion chamber charge, a high rotational supercharger speed and a large charge air mass flow into the intake plenum become apparent. If the air mass flow adjusting member is rapidly closed from such an operating condition, the air mass flow flowing out of the intake plenum will decrease very fast. Because of the inertia of the charge air flow, without countermeasures, a pressure rise will occur in the intake plenum while the charge air mass flow decreases. In this case, the flow may detach from the compressor blades and allow the air to flow back through the compressor to cause the pressure to fall.
Since, because of the high rotational energy, the rotational speed of the compressor impeller adapts only slowly to the reduced charge air demand, the flow direction reverses again after the adjustment of corresponding pressure conditions over the compressor. The process is repeated in quick succession. Because of the resulting sound, the periodic repetition of this process is also called “surging”. In order to avoid this surging, which is disadvantageous for the service life of the compressor impeller and for the noise comfort, the intake plenum is relieved by a temporary opening of the circulating-air valve to the suction side of the turbocharger.
A method of the initially mentioned type operating by way of such a circulating-air valve is known from the series “Die Bibliothek der Technik” (“Dictionary of Technical Science”) Volume 103, “Exhaust Gas Turbocharger”, Moderne Industrie Publishers, D-86896 Landsberg/Lech, ISBN 3-478-93263-7, Page 47. From page 24 of the same publication, recirculation ducts are known that return a portion of the air already entered into the compressor back into the main flow in front of the compressor. As a result, the surge limit of the compressor is to be displaced to smaller volume flow rates. This results in an enlargement of the useful area of the characteristic compressor diagram. EP 1 275 852 A2 shows what is also known as characteristic diagram stabilization, and reveals a controllable bypass to the compressor impeller. The bypass is to be closed as a rule and is to be opened in the following three cases:
In the case of a charging pressure increase from an operating point at a low load, the opened bypass should allow a higher rotational compressor speed.
An opening at the surge limit of the compressor should result in an expansion of the characteristic diagram as a result of a recirculation flow; particularly after a full-load acceleration with a subsequent abrupt release of the accelerator, the surging of the compressor is to be prevented.
By controlling the bypass cross-section, a load control is to take place as an alternative or in addition to a throttle valve control.
Up to now, the turbine-side control of the charging pressure in the case of Otto engines has taken place by a flap in a bypass duct by which exhaust gas was guided past the turbine as required. In diesel engines, turbochargers are also used where the turbine-side control of the charging pressure takes place by way of an adjustable turbine geometry. All exhaust gas will then flow by way of the turbine that permits a utilization of a larger portion of the exhaust gas energy and on optimized adjustment of the turbine flow cross-section for each operating point. In contrast to the bypass control, a higher efficiency of the turbocharger is achieved as a desirable consequence and thus also a higher efficiency of the internal-combustion engine. As a rule, diesel engines are operated in an unthrottled manner with a high excess of air. As a result, the described surging does not occur there.
In the future, turbochargers with a controllable turbine flow cross-section are to be used also in Otto engines. At a certain turbine flow cross-section, generally at a maximal turbine flow cross-section, such a turbocharger generates a minimal charging pressure, which in the following will be called a basic charging pressure. The basic charging pressure represents the minimal charging pressure that the turbocharger can provide in the case of the given exhaust gas mass flow.
Because turbochargers are generally designed such that they provide as much charging pressure as possible in the lowest rotational engine speed range while the exhaust gas mass flow rate is low, a high value for the basic charging pressure will necessarily be obtained at larger exhaust gas mass flows, thus at higher rotational engine speeds and/or combustion chamber charges.
There are circumstances in which the charging pressure has to be lowered in order to protect components of the internal-combustion engine from damage. Poor fuel quality can be mentioned here as an example and this leads to a knocking engine operation. The knocking tendency can be reduced by, among other things, the reduction of the charging pressure. A high basic charging pressure is therefore disadvantageous under these circumstances. In such cases, the limiting of the charging pressure could take place by triggering a turbine bypass flap. The disadvantage of this solution consists of the fact that, for special cases in which the basic charging pressure is too high, another adjusting member has to be provided for the turbine-side adjustment of the charging pressure with the resulting disadvantages with respect to space, weight and costs.
In view of this background, an object of the present invention is to provide a control of a turbocharger with an improved engine efficiency that avoids the above-mentioned disadvantages with a solution involving an additional adjusting member for a turbine-side controlling of the charging pressure.
In a method of the initially mentioned type, this object has been achieved in that, under certain operating conditions, in addition to the turbine-side control, which takes place by adjusting a turbine flow cross-section, the charging pressure is reduced by opening the circulating-air valve. The control device achieves the object in that it controls the course of the process.
By using the circulating-air valve, already present on the compressor side for reducing the charging pressure in the intake plenum in certain operating conditions, the charging pressure can be reduced there to values below the basic charging pressure of the turbine. This permits the use of a turbocharger with an adjustable flow cross-section and leads to the desired improvement of the efficiency. In this case, the circulating-air valve as a whole is used multiple times additionally to its known function for preventing the surging. This multiple utilization can eliminate an additional turbine-side adjusting member. The disadvantages connected by way of such an adjusting member are thereby completely avoided.